Compact wheel loaders are compact vehicles that have road wheels, as opposed to caterpillar tracks, and that carry as a working implement that is hydraulically powered. The implement may be a bucket carried by a lifting arm or a rotating broom for road sweeping.
Compact wheel loaders have an engine to drive a hydraulic pump to produce the hydraulic pressure required by the implement. The same engine is also used to drive the wheels, which are commonly powered by means of a hydrostatic transmission. Such a transmission comprises a variable displacement, engine-driven, hydraulic pump connected to one or more hydraulic motors, usually of fixed displacement, driving the wheels. The control of the variable displacement pump is performed by a double acting spool, spring biased into a central position and connected to the swash plate of the pump. The double acting spool has a working chamber connected to receive hydraulic fluid at a pilot pressure that is produced by an engine driven pump and therefore varies with the engine speed and the other working chamber is connected to a drain line leading to a reservoir. In this way, the transmission ratio is varied automatically with engine speed to provide a high transmission ratio at low engine speeds and progressively lower transmission ratios with increasing engine speed.
A loader, or any other work vehicle is sometimes required to move slowly while the implement requires a high rate of flow. In this case, the engine speed needs to be high to produce the desired fluid flow rate to support the load but the transmission ratio also needs to be high to avoid excessive vehicle speed. It is therefore also known to provide additionally mechanical gearing somewhere in the drive train leading from the engine to the wheels to shift between different speed ranges. The gearing may be anywhere in the drive train, for example in the hydraulic wheel motors, and/or in the valving of the hydraulic pump, and/or in the drive from the engine to the hydraulic pump. In all speed ranges, however, the vehicle speed is varied by pressing on a demand lever, such as an accelerator pedal that adjusts the engine speed.